In AM broadcasting in the United States, carrier power must be kept within legal limits, and F.C.C. regulations require periodic measurement of antenna impedance.
Required impedance measurements are generally made by a skilled technician using an impedance bridge and a signal generator. In the interval between measurements, which can be up to three years, antenna impedance is not known unless the station installs an inline impedance bridge, which requires a skilled operator. Antenna impedance is of interest to the station operators because it is a good indication of the status of the antenna system and because it is advantageous to maintain optimum antenna load on the transmitter. There is therefore a need for an instrument which will measure antenna impedance under operating conditions without requiring the services of a skilled technician, and it is one object of the present invention to provide such an instrument. It would be advantageous if the same instrument was capable of performing the periodic impedance measurements required by the F.C.C., which must be done at a number of different frequencies and therefore cannot use the transmitter as a signal source but require instead a low power signal generator. Thus it is a second object of the present invention to provide an instrument which can measure impedance both under operating conditions and with a low power signal generator replacing the transmitter as a signal source.
Carrier power measurement is presently accomplished, according to F.C.C. regulation, by methods which do not yield a number representing power directly, but instead require calculations, and involve assumptions about the parameters entering into these calculations, which assumptions are known to be inaccurate in practice. To remedy these defects, F.C.C. regulations will probably be changed to admit the legal use of direct reading power meters. Such meters are commercially available, but suffer from one or more of the following disadvantages; (a) sensitivity to modulation, which means that accurate power measurements can only be made during the short and infrequent intervals when the transmitter is not being modulated, (b) necessity to measure both forward and reverse power and obtain net power by subtraction, (c) implicit and not necessarily correct assumptions about antenna impedance, (d) lack of precision. There is therefore a need for an instrument which will directly measure carrier power under normal operating conditions including normal modulation, and it is a third object of the present invention to provide such an instrument. In the present invention, the quantities that enter into the determination of carrier power are the same as those which determine the resistive component of antenna impedance, hence the power measuring function can optionally be combined with the impedance measuring function in a single instrument.